Optical-type strain gauges have been in use for many years. The interferometer was an early optical strain gauge that required the adoption of in-situ parallel lens systems and other in-situ comparison and/or diagnostic apparatus. This structure prevents its use for monitoring strain from a disparate location.
The advent of fiber-optics and the application of optical fiber technology to optical strain gauge structures has permitted the construction of optical strain gauges with disparate location monitoring.
Early optical fiber strain gauge technology is described by Harmer, U.S. Pat. No. 4,163,397. Harmer not only discusses microbending losses in optical fibers, but also an apparatus for measuring the change in optical propagation characteristics in a light guiding structure (glass fiber) as a function of bending, whereby strain in the object can be determined. To maintain the undulating shape of the glass fiber in the Harmer gauge, the structure incorporates a series of studs or protrusions on two separate substrates which are attached to the surface to be measured for strain. When the two substrates move toward each other under strain perpendicular to the rows of studs, the microbend amplitude increases and the optical transmission drops. This arrangement is bulky, complex, and, depending on the size of the substrates, can significantly perturb the strain being measured. Also, no provision is made for the transverse sensitivity of the Harmer gauge, that is, the response of the gauge to strain parallel to the rows of studs.
A microbend fiber-optic pressure sensor is discussed by Fields et al, J. Acoust. Soc. Am., Vol. 67, No. 3, Mar. 1980; the device does not measure strain. The Fields structure uses one fiber-optic path and measures light intensity instead of optical phase shift, but requires a large, bulky, and complicated ridged pressure plate apparatus. Fields' device includes two mating ridged plates placed around a multi-mode step-index silica fiber, one end of which is illuminated by a laser and the other end monitored with a calibrated photometer. Motion is perpendicular to the axis of the fiber. A load applied to the pressure plates causes a quasi-sinusoidal distortion of the fiber. The device provides mechanically-induced amplification of light attenuation in the fiber caused by bending forces acting on the fiber; however, the size and bulkiness of the structure is simply not appropriate for a strain gauge.
Fiber-optic structures remain preferred structures for optical strain measurement from disparate locations. Butter, U.S. Pat. No. 4,191,470, shows a laser-fiber-optic interferometric strain gauge having two single-mode fibers, configured to compensate for the sensitivity of optical glass fiber structures to changes in ambient temperature. The two fibers are attached to different portions of a strainable member. It is suggested that one fiber be mounted to the top of a beam and the second be mounted to the bottom so that, in operation, one fiber is lengthened and the second is compressed. Mixing the light from each fiber will cause an interference pattern due to fringe shift which is indicative of the measure of the deflection of the beam. Butter measures the optical phase shift instead of light (intensity) amplitude. However, the measurement of the intensity of light received is less difficult than interferometric phase shift measurement.
Thus, there is an existing need for a simple fiber-optic strain gauge, which possesses the mechanical amplification of the Fields structure without the bulky and complicated ridged apparatus, and which measures the intensity of light received rather than the interferometric phase shift.